The overall goal of this proposal is to address a large gap in the literature on nonsuicidal self-injury (NSSI) by testing the developmental trajectories o engagement in NSSI in youth. This overall goal is in line with National Institute of Mental Health's (NIMH) Strategic Objective #2, to chart the course of mental disorders over the lifespan and define the developmental trajectories of mental disorders. Therefore, the proposed research aims to study developmental trajectories of NSSI in youth to inform current conceptualization on the etiology of NSSI, especially as this behavior may be included in the Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5) as a new psychiatric diagnosis (Shaffer & Jacobson, 2009). Additionally, the study of developmental trajectories of NSSI is consistent with the mission of the National Institute of Mental Health to understand trajectories of mental illness that can help determine how best to intervene. Longitudinal studies with developing youth are greatly needed to better elucidate the mechanisms and risk factors for NSSI as they change across the lifespan. Thus, the proposed study is designed to use a general community sample of youth and longitudinal data to better understand the development of NSSI engagement during youth, and determine if there are specific predictors of these trajectories. This is congruent with NIMH's Sidebar 2 to Strategic Objective 2, to work for early detection of risk factors for mental disorders. Several candidate predictors of different developmental trajectories of NSSI engagement in youth have been carefully selected (i.e. genes, chronic peer interpersonal stress, parenting behaviors) that may predict trajectories of NSSI. Finally, based on research showing differences among risks for NSSI engagement for youth of different ages and genders (Barrocas, Hankin, Abela, & Young, submitted), it is predicted that several of the proposed risks will explain any gender and age differences (i.e., mediate) in NSSI engagement trajectories. This study will further research in the area of NSSI in youth by employing a longitudinal design that measures NSSI engagement using the gold standard structured, clinical interview (i.e., SITBI; Nock, Holmberg, Photos, & Michel, 2007) for assessing NSSI engagement. A sample of 3rd, 6th and 9th graders have already been assessed for NSSI engagement through my sponsor's NIH-funded R01 (5R01 MH077195) study conducted with a general community sample of youth. Five waves of data assessed over the course of 2-years (collected every 6-months) will be used to test the developmental trajectories of NSSI engagement. The research will test clear a priori hypotheses about the developmental trajectories of NSSI engagement in youth, predictors of these trajectories and mediators for the association between gender and age and NSSI engagement.